1. Field of the Invention
The present invention relates generally to an apparatus for grinding the substrate of a hard disk used in electronic equipment such as computers. More particularly, this invention relates to a substrate grinding apparatus which simplifies high precision dress grinding of grinding wheels used therein. This invention further relates to such a substrate grinding apparatus which is capable of automatically measuring the thickness of a substrate after a grinding operation is completed and automatically operating the dressing means according to the results of measurement of the substrate thickness.
2. Description of the Prior Art
In recent years, the electronic equipment such as computers have exhibited an increasing tendency to use a hard disk drive as an auxiliary storage. This tendency has created a strong demand for production of the hard disk drives in a short period of time and at a low cost. As is well known, the substrate of a hard disk (hereinafter occasionally referred to, for brevity, as "substrate") must be ground before it is subjected to a polishing process followed by coating with a magnetic material.
A conventional grinding apparatus includes, as shown in FIG. 4, upper and lower platens 10 and 12 which are formed of rigid circular plates of stainless steel, for example, for securing thereto heavy grinding wheels 14, 16. The grinding wheels 14, 16 are annular in shape and have a central opening defined therein. The upper platen 10 is rotatably supported by a ball bearing arrangement (not shown). The conventional grinding apparatus also includes, as shown in FIGS. 4 and 5, a sun gear 18, a plurality of substrate carriers 20 (five being shown) for holding at least one substrate thereon, and an internal ring gear 22 forming a part of the lower platen 12 and having teeth on the inner surface of its rim. The substrate carriers 20 are capable of being induced to revolve around the sun gear 18 while simultaneously rotating about their own axes. These structural members 18, 20, 22 will be described below in greater detail. Two driving shafts 11 and 19 are connected to the upper and lower platens 10 and 12, respectively.
In the conventional grinding apparatus shown in FIGS. 4 and 5, there are five substrate carriers 20 and each of the five substrate carriers 20 has two substrate holding portions 24 in the form of circular recesses. When the sun gear 18 is driven to rotate, the substrate carriers 20 which are disposed between the sun gear 18 and the internal gear 22, revolve around sun gear 18 while rotating about their own axes. With this construction, when each of the substrate holding portions 24 contains a substrate, and the upper platen 10 (FIG. 4) is then lowered so as to grip the substrates between the upper grinding wheel 14 and the lower grinding wheel 16, suitable rotation of the sun gear 18 will ensure that plural substrates (ten substrates in the case shown in FIGS. 4 and 5) are ground at one time.
As time passes the grinding operations will cause blinding or jamming of spaces defined between abrasive grains of the grinding wheels and also wear abrasive grains down. This of course leads to a reduction in the grinding characteristics (cutting quality).
The grinding wheels 14, 16, therefore, require periodical dressing to restore their original sharpness or cutting quality. To this end, in the case of the conventional grinding apparatus such as shown in FIGS. 4 and 5, the thickness of the ground substrates is measured manually, and if the amount of grind (i.e., the amount of material removed by grinding) for a given period of time is less than a predetermined value, the grinding wheels are dressed in the following manner.
At least one of the substrate carriers 20 is removed and, thereafter, a circular dressing disk, which has the same diameter as the substrate carrier 20, and which has diamond particles or grains on both faces, is disposed in the grinding apparatus in place of the removed substrate carrier 20. The sun gear 18 is then rotated to cause the circular dressing disk to revolve about the sun gear 18 while rotating about its own axis. As a result, the grinding wheels 14, 16 are dressed by the circular dressing disk.
According to the conventional grinding apparatus described above, the substrate carrier 20 must be replaced with the circular dressing disk each time the grinding wheels are to be dressed. Such replacement work shuts down the operation of the grinding apparatus for a undesirably long period and, hence, considerably lowers the productive efficiency of the grinding apparatus.
In addition to the above drawback, the conventional grinding apparatus is not fully automatic. Furthermore, it is difficult to obtain the required precision over the entire surface of the grinding wheel when it is dressed with the diamond grain dressing disc, as diamond grains tend to wear unevenly.